1. Field Of The Invention
This invention relates generally to a fully distributed optical fiber strain sensor and, more particularly, to a fully distributed optical fiber strain sensor including a polarization maintaining optical fiber where strain in the fiber is measured as a result of Kerr effect losses in the fiber and the photoelastic effect.
2. Discussion of the Related Art
Strain sensors are an important and useful tool for measuring and monitoring strain levels in certain structures in order to provide inspection and flaw detection of the structures. Examples of structures that could benefit from such inspection and flaw detection include airplane wings and body structures, bridges and buildings. It has been suggested that it would be desirable to use strain sensors to inspect building and bridge structures after being subjected to violent conditions, such as earthquakes. Other areas that could benefit from such inspection include structures where human inspection is difficult or impossible, such as on space stations.
One type of strain sensor that has proven effective are optical fiber strain sensors used to detect strain in a structure. Typically, in these types of sensors, an optical fiber is appropriately affixed to or fabricated within a structure that is to be inspected or monitored for defects. In this configuration, strain in the structure is transferred to the optical fiber. An optical beam from an applicable light source, such as a laser or photodiode, is introduced into an end of the optical fiber during the inspection. The behavior of the optical beam is then monitored within the optical fiber. If no strain is on the optical fiber, then a base or calibrated optical condition is determined. If, however, a strain is on the optical fiber, then the light will be effected such that a difference between the calibrated optical condition and a strained optical condition can be assessed to determine the strain on the fiber.
There are a number of approaches that use disturbances in an optical beam traveling through an optical fiber as a method of detecting strain in the fiber. The most common approaches use integrating methods. These types of optical fiber strain sensors measure changes in the total length of the fiber. An optical beam propagating down an optical fiber will take a certain amount of time to travel from one end of the fiber to the other. A change in the length of the fiber as a result of a strain on the fiber will cause the time of propagation of the optical beam to be changed accordingly. The resulting difference measurement is the integral or average of the strain over the total path length of the fiber. In a variation of this integrating method, a reference fiber and a sensing fiber are provide in which the reference fiber is protected from strain and other effects. An optical beam is sent down the sensing fiber and the reference fiber, and an optical output from these two fibers are combined in an interference pattern. The interference pattern changes when a strain is applied to the sensing fiber. This change in the interference pattern is used to measure the change in length of the sensing fiber, and thus the strain in the sensing fiber. Because of this averaging affect, these types of optical strain sensors tend to have a very low sensitivity to localized phenomenon such as strain concentrations due to cracks, defects, or other geometrical considerations. Further, these types of averaging sensing methods generally assume that the strain in the fiber is in the uniaxial direction of the optical fiber. However, this assumption can sometimes lead to inaccurate results because the strain can be from any direction.
Another approach based on an integrating method is a method that utilizes the intensity modulation of an optical beam in the fiber that occurs from changing "microbends" in the fiber. This approach is based on the assumption that an increased strain, particularly in the transverse direction of the fiber, will increase minute bends in the fiber, thus resulting in greater light loss from the fiber. An improvement on this approach includes intentionally forming a series of small bends in the fiber to separate the fiber into a plurality of short sensing portions. If the fiber is strained in such a way that the distance between the ends of a sensing portion of the fiber increases, the bends in the fiber will tend to be reduced by the stretching action. This reduction in bend radii will decrease the light emitted from the fiber. Conversely, if the distance between the ends of the sensing portion of the fiber decreases, the bends in the fiber will increase and the light emitted from the fiber will also increase. Although the microbend sensors provide a simple solution for many applications, this method provides only approximate information about the state of strain in the structure, and suffers from low repeatability between detections.
Another approach is to utilize what has been referred to as point sensors. Point sensors are essentially integrating optical fiber sensors where the optical fiber has been shortened to the point where the length of the fiber is on the order of an electrical resistance strain gage. In one type of optical fiber point sensor, one end of the fiber is mirrored and a partially mirrored surface is spliced into the fiber such that a distance between the mirrored end of the fiber and the partially mirrored surface is a sensing region. A reference component of an optical beam that is emitted into the fiber from an end opposite to the mirrored end is reflected from the partially mirrored surface and a sensing component of the optical beam is reflected off the mirrored end. The reference component and the sensing component are combined at a detector to form an interference pattern. Changes in interference pattern can be analyzed to determine strain in the sensing region. Other variations of generating an interference pattern for strain detection in this manner are also known. Although the point sensors are applicable in many instances, a large number of strain sensors are necessary to provide a point indication of strain in a particular structure.
Another type of optical strain sensor is a "quasi-distributed" strain sensor referred to as an Optical Time Domain Reflectometry (OTDR) sensor. What is meant by distributed strain sensor is a strain sensor that is capable of determining strain at any point along the sensing region of the fiber. An OTDR sensor is a sensor that introduces short pulses of light into one end of a sensing fiber, then analyzes the portion of the pulses that are reflected back from several strategically located prefabricated markers in the fiber. The prefabricated markers are generally made by cleaving the fiber, then reattaching the fiber in an imperfect manner. Reflection spikes from the marker points in the fiber are displayed on a screen to show the distance between the markers along the fiber. A change in the calibrated distance between the spikes represents a strain in the optical fiber at that location.
Although the OTDR sensors provide advantages over the integrating sensors, the OTDR sensors still suffer from a number of disadvantages. These disadvantages include a limitation on the number of marker points because of losses of the signal pulse energy and resolution of the system, the high cost of the precision equipment needed for the measurements, slow speed of measurements that result from the low energy signal and the large number of averages needed to gain the required precision, the result of prefabricating the markers before the sensor is installed in the structures, and the need for expert operator to read and interpret the spike signal data.
Advances in fiber optic strain sensors have led to sensors that provide fully distributed sensing. That is, the sensor can indicate strain at any location along a sensing portion of the fiber. Different fully distributed fiber optical sensors that use polarization maintaining (PM) fibers, and make use of the Kerr effect to determine strain are known. A polarization maintaining fiber is a fiber that will maintain the direction of polarization of an optical beam that is introduced into the fiber. The Kerr effect is a nonlinear optical dependence on the intensity of light that effects the refractive index of an optical material. The Kerr effect is a well understood phenomenon in optics, and will be discussed in more detail below with reference to the preferred embodiments.
One type of optical fiber strain sensor that utilizes PM fibers and the Kerr effect is referred to as frequency-derived backscatter, as disclosed in Rogers, A. J. et al, "Novel Methods for Distributed Optical Fibre Sensing", proceedings of the SPIE, September, 1991 (Boston). The frequency-derived backscatter method includes measuring the spacial distribution of birefringence in a highly birefringent fiber. Strong optical pulses are introduced into the PM fiber with equal energy for each of the polarization directions of the fiber. Ideally, the pulse length should be one half of the "beat length" of the PM fiber. A back-reflected signal from the opposite end of the fiber is monitored by a photodiode as a function of time for each polarization direction. As a result of the changing polarization state of the pulses as they travel down the fiber, there will be moderately high frequency modulation of the backscattered signal. By sampling simultaneously at two wavelengths, there is an additional higher frequency output signal that can be used to effectively separate the spatial changes in polarization.
Another approach similar to the frequency-derived backscatter appoach is referred to as forward-scatter frequency -derived distributed optical fiber sensing as disclosed in Parvaneh, F. et al, "Frequency-Derived Remote Measurement of Birefringence in Polarization-Maintaining Fiber by Using the optical Kerr Effect", Optics Letters, Vol. 17, No. 19, Oct. 1, 1992, pp. 1346-1348. This method uses strong optical pulses introduced into one end of a fiber at 45.degree. relative to a polarization maintaining axis of the fiber. The polarization state of the pulses will then change cyclically through all states of polarization due to the different propagation characteristics in the major and minor directions of the PM fiber. If the pulse is of sufficient power to produce a significant Kerr effect, the Kerr induced change in index will have the effect of causing the polarization axis of the fiber to oscillate sinusoidally down the fiber. If a continuous optical signal is counterpropagated from an opposite end of the fiber and is aligned with one of the polarization axis of the fiber, when the continuous signal encounters the pulse travelling in the opposite direction, a portion of the energy will get cross-coupled into the other polarization direction as a result of the oscillating polarization axis. An emerging signal of the continuous signal will contain a spatially mapped pattern representing the axial polarization state of the fiber. Any change in the axial polarization state relative to the original, non-strained state can then be distinguished.
Another approach similar to the frequency-derived backscatter approach is referred to as optical Kerr effect frequency-derived backscatter using measure-and-induced coupling. This approach introduces an optical pulse along one of the polarization axis of a PM fiber, and introduces a continuous optical beam at 45.degree. relative to the polarization axis. A variation of this approach includes introducing a continuous optical beam into the fiber at one end equally exciting both polarization axis of the fiber. A strong pulse of a different wavelength is introduced into the fiber from the other end, also equally exciting both polarization axis of the fiber. The result of this arrangement is a modulation of the pulse signal at a frequency that is a function of the birefringence of the fiber and a difference in wavelength between the pulse and the continuous beam. This modulation can be used to measure the birefringence distribution over the length of the fiber.
Although fully distributed optical fiber sensors provide significant advancements in determining strain in an optical fiber, these types of sensors still do not offer the type of anisotropic measurement that allows a determination of strain in all directions of the fiber.
What is needed is a fully distributed optical fiber strain sensor that allows fully distributed strain sensing, and provides a measurement of strain in all directions. It is therefore an object of the present invention to provide such a strain sensor.